1. Field of the Invention
This invention relates to a solid magnetic memory and more particularly to a Bloch line memory device suitable for accomplishing a large capacity file memory.
A Bloch line memory is memory means that has drawn an increasing attention in the fields of information, communication, measurement, and the like, as a magnetic memory which will improve drastically the high density of magnetic bubble memory which is a non-volatile magnetic memory, too. The Bloch line memory device in accordance with the present invention can also be applied suitably for a data memory device of an electronic switchboard, a numeric control machine tool, a personal computer, POS, medical equipment, satellite, and so forth.
2. Description of the Prior Art
The Bloch line memory device uses a magnetic garnet as a memory medium in the same way as the magnetic bubble memory device, but its memory system is remarkably different from that of the latter. In the conventional magnetic bubble device, the existence and absence of bubble domains are made to correspond to "1" and "0" of information. In the Bloch line memory device, on the other hand, the existence and absence of a vertical Bloch line pair existing inside the wall around a stripe domain formed by extending the bubble domain is made to correspond to "1" and "0".
Hereinafter, the principle of the Bloch line memory will be explained.
FIG. 4 shows schematically the stripe domain in the magnetic film in the Bloch line memory device and the structure around its periphery. In the drawing, upward arrow 413 in the stripe domain 402 represents the direction of magnetization in the domain 402. Similarly, arrow 411 on the center line in the magnetic wall 401 represents the direction of magnetization located at the center of the wall 401 and arrow in a vertical direction to the center line of the magnetic wall represents a vertical Bloch line 403 (which will be hereinafter referred to merely as the "Bloch line"). The Bloch line memory stores the information by letting the portion 404a, where two Bloch lines exist as a pair, correspond to the information "1", for example, and the portion 404b, where they do not exist, to the information "0".
As described above, the Bloch line used as the information carrier is a very fine magnetization structure that exists in the magnetic wall 401 surrounding the stripe domain 402. The Bloch line exists stably in the magnetic wall 401 and can move freely inside it. Therefore, if a large number of stripe domains 402 are disposed in parallel at a predetermined position and Bloch lines 403 are allowed to exist in the magnetic wall thereof, they exhibit the behaviour just in the same way as the bubble domain that moves in the minor loop of a magnetic bubble memory device. For this reason, the Bloch line memory device can assume a shift register type memory device structure in the same way as the magnetic bubble memory device.
The presence of the Bloch line has long been known and experiments and their analysis have evidenced that the moving velocity of the domain where the Bloch line exists becomes smaller than that of the domain not having the Bloch line. Accordingly, in the magnetic bubble memory device requiring the movement of the domain, the bubble domain containing the Bloch line is referred to as a "hard bubble" and contrivances have been made so as to prevent its occurrence. In contrast, the Bloch line memory device utilizes positively the existence of this Bloch line.
The physical size of the Bloch line is about 1/10 of the width of the stripe domain where it exists and a large number of Bloch lines can be made to exist in one stripe domain. In the case of a magnetic garnet having a stripe domain width of 1 .mu.m that has been developed at present for a magnetic bubble memory device, for example, about 5.times.10.sup.8 Bloch lines can be made to exist per 1 cm.sup.2. Therefore, if an information carrier is prepared by use of two Bloch lines as a pair, a memory device having a memory capacity of a 256 Mbit/cm.sup.2 class can be fabricated.
As described above, the Bloch line can freely move around the stripe domain wall and can store the information (has a memory function). However, the write function and the read function of the information must be accomplished in order to constitute a memory device.
To attain the write function, a system has been known generally which causes a current to flow through a conductor disposed at the end of the stripe domain and applies a local field to the end portion of the stripe domain so as to invert the field by 180.degree.. In other words, this can be understood by regarding that part of the area of the magnetization state 404b represented by "0" in FIG. 4 is inverted and attains the state 404a of the "1" area. At this time the boundary between the inverted area and the non-inverted area generates the state which has changed by 90.degree. with respect to the magnetic wall because magnetization changes continuously. This is the Bloch line 403. Incidentally, since this state is generated always while two Bloch lines 403, 403' exist as a pair, binary information can be made to correspond to the existence and absence of a pair of Bloch lines.
The read operation of the information is conducted by transferring the existence and absence of the Bloch lines to the existence and absence of bubble domains. A transfer method from the Bloch line to the bubble domain is described by Konishi in IEEE Transactions on Magnetics, MAG-19, No. 5, 1983, pp. 1838-1843. In FIG. 4, if the Bloch line 403 exists in the wall 401 of the stripe domain 402, the direction of magnetization in the wall 401 inverts, with the Bloch line 403 being the boundary. Due to such a change of the magnetization structure, a change occurs in the easiness of chopping out the end portion of the magnetic domain between the case where one Bloch line 403 comes to move to the end portion of the stripe domain 402 and the case where no Bloch line 403 exists at the end portion. It is possible by utilizing this property to chop a bubble magnetic domain from the end portion of the stripe domain 402 only when one Bloch line 403 exists at the end portion of the stripe domain 402 by causing a predetermined current to flow through a chopping conductor disposed on and near the end portion of the stripe domain 402. The bubble domain thus chopped is transferred by the same method as the measure line of the bubble memory device and converted to an electric signal and by so doing, the existence of the Bloch line can be read out.
As described above, the Bloch line memory device can be accomplished by forming the functional portions of the write, store and read operations on the same device.
The write and read operations described above are disclosed, for example, in Japanese Patent Laid-Open No. 151374/1984.
FIG. 5 is a plan view of the Bloch line memory device. In this drawing the information storage portion is formed by aligning a large number of stripe domains 502. A transfer pattern 507 for stably transferring the Bloch line pairs and holding them are arranged, in such a manner as to cross orthogonally the stripe domains 502. The transfer pattern is formed by etching selectively etching the surface of a magnetic material or a magnetic garnet. A read functional portion 521 is disposed on the right side of the device while a write functional portion 520 is disposed on the left side.
The write functional portion 520 writes the Bloch line pair in the manner described above. If it is not desired to write (or to write "0" when the Bloch line pair is made to correspond to "1", for example) to a given stripe domain, a magnetic bubble 510 is caused to exist at a position on the extension of the end portion of that stripe domain. If the magnetic bubble exists; its magnetostatic repulsive force acts on the end portion of the stripe domain and the end portion of the stripe domain to which the write operation is not desired to be made can be separated from the write gate (in the rightward direction in the drawing) as shown in the drawing. In this manner the point of intersection between the conductor constituting the write functional portion 520 and the stripe domain can be released and the Bloch line pair is not written into the stripe domain even if the current pulse is applied to the conductor for the purpose of writing. (In other words, "0" is written in the embodiment described above). In order to provide the function described above, a magnetic bubble transfer path 523 and a magnetic bubble generator 512 are disposed in the write functional portion 520.
On the other hand, the read operation in the Bloch line memory device is conducted in the read functional portion 521. The existence of the magnetic bubble chopped out in the read operation is converted to the electric signal by the magnetic bubble detector 511 through the read transfer path 522. The existence and absence of this electric signal correspond to the existence and absence of the magnetic bubble or in other words, to whether or not the Bloch line exists at the end portion of a predetermined stripe domain. In this manner the read operation can be carried out.
In order to improve stability of the Bloch line pair, Japanese Patent Laid-Open No. 101092/1984 discloses a method which applies a bias magnetic field in an in-plane direction of a magnetic film and bringing the magnetization state of the stripe domain into an S=0 state. Here, symbol S is an S number which represents the number of revolutions of wall center magnetization when the domain turns once round the magnetic wall. If the Bloch line pair is kept stable by this method, three Bloch lines exist at the stripe domain head if the information "1" exists there and one Bloch line exists if "0" exists, when the information "1" is made to correspond to the "existence" of the Bloch line pair, for example.
According to this method, the Bloch line exists always at the stripe domain head irrespective of the information "1" and "0" and the information read operation cannot be made by the method described already alone.
Japanese Patent Laid-Open No. 248296/1986 discloses a method which solves this problem and accomplishes the read operation. This method comprises disposing two read preparation conductors at the end portion of a ring shaped domain, applying a transfer magnetic field and the causing a predetermined current pulse to flow through these conductors. According to this operation, one Bloch line can be made to exist at the stripe domain head only when the information "1" is transferred, and its existence and absence can be converted to those of the bubble and can be read out by the method of Konishi described already.
The method described above conducts the preprocessing of the read operation by utilizing the transfer magnetic field. In consequence, one Bloch line that exists at the stripe domain head in the case of the information "0" always moves in the transfer direction. Therefore, if the transfer direction is in the leftward direction towards the chopping conductor from the other end of the stripe domain as viewed from the film surface on which the chopping conductor is disposed, or in other words, if the transfer direction of the Bloch line is left turn as viewed from the film surface on which the chopping conductor is disposed, magnetic wall magnetization at the stripe domain head faces leftward when the read preparation operation is conducted. Accordingly, since the magnetic wall magnetization faces leftward, from the other end of the stripe domain towards the chopping conductor, or in other words, in the direction of the left turn described above, as viewed from the film surface on which the chopping conductor is disposed, the horizontal Bloch lines occur from the upper surface of the film equipped with the chopping conductor.